The present invention relates to a novel method for the preparation of an organopolysiloxane having a branched molecular structure as a class of silicone products, which is useful in a great variety of industrial application fields.
Among the great variety of silicone products, as is well known, those organopolysiloxanes prepared by utilizing the so-called hydrosilation reaction are particularly important and have usefulness in a wide field of applications. The above mentioned hydrosilation reaction is conducted usually between an alkenyl group-containing organopolysiloxane and an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms in the presence of a catalytic amount of a platinum compound to promote the addition reaction.
The alkenyl group-containing organopolysiloxane pertaining to the hydrosilation reaction is not particularly limitative relative to the molecular structure depending on the desired products. When an alkenyl group-containing organopolysiloxane having a branched molecular structure is used, a product organopolysiloxane of good reactivity containing a controlled amount of the alkenyl groups at the molecular chain ends to comply with the requirements in different applications can be prepared by adequately selecting the number of branches in the starting alkenyl group-containing organopolysiloxane (see Japanese Patent Publication 3-19267 and Japanese Patent 2965231). However, only very few reports are available on the method for the preparation of such an alkenyl-terminated organopolysiloxane of a branched molecular structure and the only methods thus far disclosed are each very complicated to inhibit practical application of the method.
For example, Japanese Patent Publication 3-19267 proposes a method in which an alkyl trimethoxy silane and octamethyl cyclotetrasiloxane are subjected to a polymerization reaction in the presence of an alkaline catalyst and then this polymer is subjected to a hydrolysis-condensation reaction with tetramethyl divinyl disiloxane under an acidic condition and a method in which an alkyl trimethoxy silane is subjected to hydrolysis-condensation with tetramethyl divinyl disiloxane under an acidic condition followed by a polymerization reaction thereof with octamethyl cyclotetrasiloxane in the presence of an alkaline catalyst. These methods, however, are disadvantageous because, in addition to complicacy of the method as an industrial process, the viscosity of the final product can hardly be controlled with a large variation as a consequence of the alkaline polymerization in the presence of a large amount of the alkoxy groups and the reaction mixture is sometimes under a danger of bumping due to the water and/or alcohol contained therein.
The present invention accordingly has an object to provide, in view of the above described problems and disadvantages in the prior art methods, a novel and efficient industrial method for the preparation of an organopolysiloxane having a branched molecular structure from starting materials of good availability.
Thus, the present invention provides a method for the preparation of an organopolysiloxane having a branched molecular structure which comprises the steps of:
(A) Mixing
(A1) from 1 to 100 parts by weight of a first organopolysiloxane represented by the average unit formula
(R13SiOxc2xd)m(R1SiO{fraction (3/2)})n[R1(R2O)SiO]q, xe2x80x83xe2x80x83(I)
in which R1 is, each independently from the others, a monovalent hydrocarbon group having 1 to 8 carbon atoms selected from the group consisting of alkenyl groups, alkyl groups and phenyl group, R2 is a hydrogen atom, methyl group or ethyl group and the subscripts n, and q are each 0 or a positive number with the proviso that (n+q)/m is in the range from 0.6 to 1.5 and q/(m+n) is 0 or a positive number not larger than 0.05;
(A2) from 1 to 100 parts by weight of a cyclic dialkylsiloxane oligomer, and
(A3) from 1 to 100 parts by weight of a polydiorganosiloxane represented by the structural formula
R33Sixe2x80x94Oxe2x80x94(SiR32xe2x80x94O)rxe2x80x94SiR33, xe2x80x83xe2x80x83(II)
In which R3 is, each independently from the others, a monovalent hydrocarbon group having 1 to 8 carbon atoms selected from the group consisting of alkenyl groups, alkyl groups and phenyl group and the subscript r is 0 or a positive integer not exceeding 100, to give a siloxane mixture;
(B) admixing the siloxane mixture with an alkaline compound; and
(C) heating the siloxane mixture to effect siloxane rearrangement polymerization of the organopolysiloxanes (A1), (A2) and (A3).
In particular, it is preferable that at least one of the three R1 groups in one of the siloxane units (R13SiOxc2xd) in the organopolysiloxane (A1) is an alkenyl group.
The starting material in the method of the present invention is a combination of three kinds of organopolysiloxanes (A1), (A2) and (A3) mixed together in a specified mixing proportion to give a siloxane mixture. The organopolysiloxane (A1) is represented by the average unit formula (I) given above, in which each R1 is, independently from the others, a monovalent hydrocarbon group having 1 to 8 carbon atoms selected from the group consisting of alkenyl groups such as vinyl, allyl, butenyl and pentenyl groups, alkyl groups such as methyl and ethyl groups and phenyl group and the subscripts m, n and q are each 0 or a positive number with the proviso that (n+q)/m is in the range from 0.6 to 1.5 and q/(m+n) is 0 or a positive number not larger than 0.05.
When the value of (n+q)/m is smaller than 0.6, the number of the branching points in the branched-structure organopolysiloxane prepared by using such a starting organopolysiloxane is undesirably limited to 3 or less to cause a decrease in the usefulness of the product in applications. When the value of (n+q)/m is larger than 1.5, the molar proportion of the units R1SiO{fraction (3/2)} is so large as to cause a difficulty in the preparation of the product organopolysiloxane. When the value of q/(m+n) is larger than 0.05, the organopolysiloxane has a too large number of remaining alkoxy groups to cause instability or uncontrollability of the viscosity thereof.
The organopolysiloxane (A1) can be prepared by subjecting a mixture of a trialkoxy methyl silane, dialkenyl tetramethyl disiloxane and hexamethyl disiloxane in an alcoholic solution to a cohydrolysis reaction in the presence of an acidic catalyst followed by neutralization of the catalyst, removal of the by-product alcohol, washing with water and stripping of the unreacted reactants.
The second organopolysiloxane reactant (A2) pertaining to the reaction of the inventive method is a cyclic dialkylsiloxane oligomer including those represented by the general formulas [Me2SiO]t, [EtMe-SiO]t and [PrMeSiO]t, in which Me is a methyl group, Et is an ethyl group, Pr is a propyl group and the subscript t is a positive integer of 3 to 10. These cyclic dialkylsiloxane oligomers can be used either singly or as a combination of two kinds or more.
The third organopolysiloxane reactant (A3) pertaining to the reaction of the inventive method is a diorganopolysiloxane having a linear molecular structure as represented by the above given general formula (II), in which each R3 is, independently from the others, a monovalent hydrocarbon group having 1 to 8 carbon atoms exemplified by alkenyl groups such as vinyl, allyl, butenyl and pentenyl groups, alkyl groups such as methyl and ethyl groups and aryl groups such as phenyl group and the subscript r is 0 or a positive integer not exceeding 100.
The mixing proportion of the three organopolysiloxane reactants (A1), (A2) and (A3) is selected depending on several parameters characterizing the desired organopolysiloxane product having a branched molecular structure such as the average degree of polymerization and number of the branching points in a molecule to meet the intended application. Namely, the amount of each of the reactants (A1) to (A3) can be in the range from 1 to 100 parts by weight without particular limitations.
The reaction of the above described reactant organopolysiloxanes according to the inventive method is carried out in the presence of an alkaline compound as a catalyst to promote the reaction. Suitable alkaline compounds as the catalyst include alkali metal hydroxides such as potassium hydroxide and sodium hydroxide as well as the reaction products between an alkali metal hydroxide and a dimethylpolysiloxane prepared separately. The amount of the alkaline catalytic compound is selected in such a way that the molar ratio of the alkali metal in the catalytic compound to the silicon atoms in the organopolysiloxanes is in the range from 10xe2x88x925 to 10xe2x88x923.
The siloxane rearrangement polymerization reaction of the three reactant organopolysiloxanes according to the inventive method is conducted at a temperature of 100 to 180xc2x0 C. taking 4 to 12 hours. The organopolysiloxane mixture can be diluted by the addition of an organic solvent which can be selected from the organic solvents conventionally used in the siloxane rearrangement polymerization of organopolysiloxanes without particular limitations. It is further optional that the reaction mixture is admixed with an organopolysiloxane of a different type than the above described (A1) to (A3).
The method of the present invention is described in the following in more detail by way of Examples, which, however, never limit the scope of the invention in any way, as preceded by the description of the preparation procedures of the respective reactant organopolysiloxanes. The term of xe2x80x9cvinyl equivalentxe2x80x9d appearing in the following refers to the number of moles of the vinyl groups contained in 100 g of the organopolysiloxane. The values of viscosity are all the values obtained by the measurement at 25xc2x0 C. and the viscosity values of the product organopolysiloxanes are given in the unit of MPaxc2x7s.
An organopolysiloxane, referred to as the organopolysiloxane (a) hereinafter, satisfying the definition of the organopolysiloxane (A1) was synthesized in the following manner. Thus, a reaction mixture obtained by mixing 544 g of methyl methoxy silane, 372 g of tetramethyl divinyl disiloxane and 200 g of isopropyl alcohol was admixed with 9.0 g of methanesulfonic acid and then 129.6 g of water were added dropwise into the reaction mixture at such a rate that the temperature of the reaction mixture never exceeded 70xc2x0 C. to effect cohydrolysis of the silane and siloxane compounds. The reaction mixture was then neutralized by the addition of 16 g of sodium hydrogencarbonate and the methyl alcohol formed as a by-product was removed by distillation under normal pressure followed by washing with water and stripping of residual volatile matters to give 490 g of a clear liquid organopolysiloxane having a viscosity of 30 centipoise. This organopolysiloxane (a) was analyzed by the methods of NMR, IR and GPC and could be identified to be expressed by the average unit formula
[ViMe2SiOxc2xd]1.0[MeSiO{fraction (3/2)}]1.11[Me(MeO)SiO]0.05,
in which Me is a methyl group and Vi is a vinyl group.
Another organopolysiloxane, referred to as the organopolysiloxane (b) hereinafter, satisfying the definition of the organopolysiloxane (A1) was synthesized in the following manner. Thus, a reaction mixture obtained by mixing 544 g of methyl methoxy silane, 324 g of hexamethyl disiloxane and 200 g of isopropyl alcohol was admixed with 9.0 g of methanesulfonic acid and then 129.6 g of water were added dropwise into the reaction mixture at such a rate that the temperature of the reaction mixture never exceeded 70xc2x0 C. to effect cohydrolysis of the silane and siloxane compounds. The reaction mixture was then neutralized by the addition of 16 g of sodium hydrogencarbonate and the methyl alcohol formed as a by-product was removed by distillation under normal pressure followed by washing with water and stripping of residual volatile matters to give 430 g of a clear liquid organopolysiloxane having a viscosity of 20 centipoise. This organopolysiloxane (b) was analyzed by the methods of NMR, IR and GPC and could be identified to be expressed by the average unit formula
[Me3SiOxc2xd]1.0[MeSiO{fraction (3/2)}]1.01[Me(MeO)SiO]0.07,
in which Me is a methyl group.
A third organopolysiloxane, referred to as the organopolysiloxane (c) hereinafter, which was similar to the organopolysiloxanes (a) and (b) but did not fall within the definition of the organopolysiloxane (A1) was synthesized in the following manner. Thus, a reaction mixture obtained by mixing 544 g of methyl methoxy silane, 324 g of tetramethy divinyllyl disiloxane and 200 g of isopropyl alcohol was admixed with 9.0 g of methanesulfonic acid and then 108 g of water were added dropwise into the reaction mixture at such a rate that the temperature of the reaction mixture never exceeded 70xc2x0 C. to effect cohydrolysis of the silane and siloxane compounds. The reaction mixture was then neutralized by the addition of 16 g of sodium hydrogencarbonate and the methyl alcohol formed as a by-product was removed by distillation under normal pressure followed by washing with water and stripping of residual volatile matters to give 495 g of a clear liquid organopolysiloxane having a viscosity of 25 centipoise. This organopolysiloxane (c) was analyzed by the methods of NMR, IR and GPC and could be identified to be expressed by the average unit formula
[ViMe2SiOxc2xd]1.0[MeSiO{fraction (3/2)}]1.16[Me(MeO)SiO]012, 
in which Me is a methyl group and Vi is a vinyl group.